Impact of long term exposure to food-associated stresses on the virulence potencial of Listeria monocytogenes Strains

Abstract

Human listeriosis, caused by the consumption of food contaminated with Listeriamonocytogenes, is on the top five most commonly reported zoonosis under the surveillance ofthe European Union (EU), and presents the highest fatality rate. This foodborne pathogen isparticularly problematic for the food industry because it is widespread in the environment,and because of its ability to survive under several stress conditions such as refrigerationtemperatures, high salt concentrations, low water activity (aw), or a wide pH range. Exposureto a single or multiple sublethal stresses, as those impaired by food processing and foodmatrices, can enhance tolerance of Listeria monocytogenes to stresses and increase its survivaland pathogenesis. This knowledge is needed to develop efficient control strategies to improvefood safety. The main objective of this study was to investigate and compare the impact ofexposure to stress conditions, frequently encountered in food-associated environments, on thevirulence potential of eight L. monocytogenes strains, including strains EDG-e, Scott A andLm 2542, associated with a listeriosis outbreak occurred in Portugal. Strains were grown inBrain Heart Infusion (BHI) medium (i) at low temperature (11 ºC, cold stress), (ii)supplemented with 6% NaCl (w/v) at low temperature (11 ºC; cold-osmotic stress), and (iii) atstandard conditions (37 ºC), and tested for their ability to invade the human intestinalepithelial Caco-2 cells. Our results demonstrated that long-term exposure to cold stressconditions contributed for enhanced invasion efficiency of the four tested strains whencompared to cells grown at 37 ºC, while no differences were observed when cells were grownat high salt concentrations. Further experiments evaluated the effect of exposure to thesestress conditions on the survival of three selected L. monocytogenes strains through an in vitrogastrointestinal (GI) tract digestion model, using Ultra High Temperature (UHT) milk as foodmatrix, and subsequent invasiveness potential. The exposure to cold-osmotic stress increasedthe survival of one L. monocytogenes strain (Lm 2542) through the GI tract, that subsequentlypresented a significantly higher invasion efficiency. More studies are necessary for a betterunderstanding of the mechanisms that overlap between adaptation to stress improving and anincrease in virulence-related characteristics in these specific strains of L. monocytogenes.

Date of Award	27 Apr 2018
Original language	English
Awarding Institution	Universidade Católica Portuguesa
Supervisor	Paula Teixeira (Supervisor) & Vânia Ferreira (Co-Supervisor)